Why is the Al101 centered at a frequency of 40 kHz?
There are three ranges of sound with respect to what humans can or cannot hear: infrasonic, audible, and ultrasonic. Sounds below 20 Hz are infrasonic and cannot be heard by the human ear. Sources of infrasound include severe weather, volcanoes, and seismic activity. Humans can hear sounds between 20 Hz and 20,000 Hz (20 kHz). Once a mechanical defect can be heard at this audible level, damage to the component and possibly to the surrounding components has already occurred. Any sound with a frequency above 20 kHz is ultrasonic.
